Many of the conventionally-known automotive two-wheeled vehicles or motorcycles employ a structure where a seat rail extends from a vehicle body frame and a seat is supported on the seat rail. Among the conventionally-known motorcycle seat rails are ones made of pipes, plates, etc. as well as ones made by casting as disclosed, for example, in Japanese Patent Application Laid-Open Publication No. 2007-261587 (“patent literature 1”).
As disclosed in patent literature 1, a seat frame, which constitutes the seat rail, is made by die-casting and extends obliquely rearward and upward from a rear arm bracket that is a rear-part member of the vehicle body frame.
The seat frame comprises left and right rail sections interconnected at their upper ends by a cross member. Each of the rail sections includes a large-thickness base portion, an extension portion extending from the large-thickness base portion, and a large-thickness edge portion formed at the lower end of the extension portion.
The seat frame has a generally U sectional shape opening downward, and thus, it can be readily released from a mold; particularly, the seat frame has a sectional configuration suited for die-casting. As loads of a passenger and goods onboard act downwardly on the seat frame, stress would increase in the edge portion. As measures against such a stress increase, the edge portion is formed to have a large thickness as noted above in order to provide an increased sectional area so that the stress increase can be limited to below a predetermined level. Namely, it is effective to increase the thickness of the edge portion.
So far, the dominating seat frames of this type have been ones made by die-casting an aluminum alloy (i.e., aluminum-alloy die-cast seat frames), because such an aluminum alloy can achieve a lighter weight than an iron-based alloy.
In recent years, more and more seat frames made by die-casting a magnesium alloy (i.e., magnesium-alloy die-cast seat frames) are being brought into practical use, for purposes of further weight reduction etc. Magnesium has a high specific strength and superior workability and vibration absorbing performance as compared to aluminum. However, because a magnesium alloy is easily subject to oxidation, it took a long time to establish a technique for making a seat frame by die-casting a magnesium alloy, and thus, practical realization of a seat frame made by die-casting a magnesium alloy has been delayed until recent years.
The following explain specific gravities and Young's moduli of such an aluminum-alloy die-cast seat frame and magnesium-alloy die-cast seat frame, with reference to Table 1 below.
TABLE 1Name ofSpecificYoung'sAlloyGravityModulusAluminum-alloyADC32.770 GPaDie-castMagnesium-alloyAM60B1.844 GPaDie-cast
The magnesium alloy is lighter in weight than the aluminum alloy but much smaller in Young's modulus than the aluminum alloy. Because the Young's modulus influences an amount of bending or deflection, the magnesium alloy bends more greatly than the aluminum alloy in response to a same load. To limit the amount of bending to below a predetermined value, it is necessary to increase second moment of area. For that purpose, there is a need to further increase the thickness of the above-mentioned edge portion. However, if the thickness of the edge portion is increased, the seat frame would increase in overall weight and size, so that manufacturing cost of the seat frame would increase.
Further, if the edge portion is formed to project horizontally, water tends to accumulate on/in the upper surface of the horizontally projecting edge portion. Water accumulation on/in the upper surface of the horizontally projecting edge portion is undesirable in that corrosion is likely to occur in the edge portion. Such water accumulation is also undesirable from the viewpoint of operability.
Therefore, there has been a demand for a motorcycle seat rail where water is difficult to accumulate and which has a sufficient rigidity.